BoilerFaultCodesMain E118 Fault Code
The Main E118 fault code means: your boiler has stopped working because the water pressure in the system has dropped too low and needs topping up.. It appears on 2 Main models in our database.
Technical description: Primary system water pressure too low
What causes the Main E118 fault?
This fault occurs when the pressure within your central heating system drops below a safe operating level, usually because water has escaped the circuit. The most common reasons for this are a leak in the radiator valves or pipework, or air being released from the system through recent bleeding of the radiators.
This fault is frequently seen in autumn when homeowners turn their heating back on for the first time after summer, revealing pressure drops that occurred during the warmer months.
DIY fix possible
Locate the filling loop (usually two braided silver hoses or blue levers) underneath the boiler.Open the valves slowly until you hear water entering the system.Watch the pressure gauge and close the valves tightly once the needle reaches 1.5 bar.
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Gas Safe Register →Main models that show the E118 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Main E118 fault code information on this page is sourced and cross-checked against the official Main service / installation manuals for each affected model.
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026